Metal honeycombs for exhaust system applications are made from metal powders which are mixed together, extruded and then sintered to the desired density. Such components can be made of various extruded elements which are joined together in the green (extruded and dried, but not fired) state and then sintered together to an integral structure. For example, preheaters are made from a metal honeycomb body with two rods or bars joined at the ends to serve as electrodes. When a green part is placed in the furnace for sintering, it must be supported by sacrificial pieces of extruded metal which allow for the shrinkage that takes place during densification (up to about 20 linear % for some bodies) without distortion of either the fine honeycomb structure or the attached pieces (if there are any). In addition, sacrificial pieces are used to insulate the part from unwanted interactions with contaminants in the furnace gas.
The sacrificial pieces must be removed after firing. However, extruded metal parts that are in contact during the sintering process tend to sinter together (bond) making their separation difficult or impossible. Thus, sacrificial pieces can become joined to the fired parts during sintering and are not removable without damage to both.
Release or parting agents can be used to keep parts from sintering together. Applied to one or both of the mating surfaces, the release agent prevents bonding between the surfaces themselves and/or between the surfaces and the release agent. Therefore, the release agent must keep the surfaces apart and be unreactive with the material of which the surfaces are made. In the case of greenware, that is complicated by the fact that physical and chemical reactions take place during sintering that change the nature of the surfaces which are to be kept apart. Furthermore, there must be no reactivity throughout the entire temperature range of the sintering process.
In the case of an FeCrAl alloy, the release agent must not form any reaction product with any of the batch components throughout the sintering temperature range. Specifically with respect to oxygen, the presence of aluminum in the alloy means that if the release agent is an oxide, it must be as stable as, or more stable than, alumina and have no reactivity with alumina. For example, silica-containing powders can not be used because the silica will be reduced by aluminum during the heating process. Some commonly used release agents such as molybdenum and graphite powder or spray cannot be used because of unwanted reactions with the metal bodies.
Another release agent is boron nitride spray which works well to keep parts from sintering together, but reacts with the fired metal bodies during sintering and/or subsequent oxidation steps, leading to accelerated oxidation. Since excellent oxidation resistance is a critical attribute for some applications (e.g., exhaust system components), degradation of oxidation resistance is an unacceptable price to pay for a release agent.
Loose alumina powder can be used to keep parts from bonding together during sintering, but it can only be applied to surfaces that are "up" with respect to the furnace hearth plate (i.e., top surfaces of the parts) during firing. Also, loose alumina powder can be released inside the furnace with detrimental effects.
The release agent must not cause cell deformation during sintering-induced shrinkage. Such deformation can result from a powder release agent with excessively coarse grain size in which case parts ride on individual grains, exerting excess stress on thin and fragile honeycomb surfaces.
Accordingly, there remains a need to improve upon release agents that are used in sintering of metal parts which will cause separation of the parts without damage to the parts, and which will not adversely affect oxidation resistance.
The present invention provides such a release agent.